Nest 3rd gen - hot water issue

[vikdab]

Hello

I have a nest third generation thermostat that was connected five years ago, The heat link died and google offered to replace for free. Around the same time another engineer replaced a mid position valve. I believe my system is neither s nor y plan (it has a mid position valve and a 2 position valve that connects to a megaflow)

I noticed that I was having issues with hot water. The guy who changed the mid position valve assures me that the wiring is correct for the valves (he doesn't know how to wire nest). Boxt engineer (who seems clue less to me says there is nothing wrong with the nest wiring (I suspect he is wrong)

Issue I have is that hot water does not work when heating is on. I have to switch off heating to get hot water. When heating is on and I turn hot water on boost I can see the 2 position valve is opening (the lever on back is free) so I suspect that there is something wrong with the mid position valve.

I have attached photo of nest wiring.

Please help

 

[muffking]

Did it work ok when the Nest Heat Link was replaced before the 3-port valve was changed?
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I suspect you have a Y-Plan system with an additional 2-port valve to protect the cylinder from turning into a kettle in the event of a fault.
See:- https://flameport.com/electric/central_heating/heating_wiring_Y_plan_unventedHW.cs4
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The 3-port valve will stop HW from flowing when CH is ON if it's been told to move to Port-A only. You can check what the position of vale should be using the following test sheet.

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The only diagram you really need to check the NEST is this.
Terminal 6 should be live when Hot Water in ON, 4 should be Live when Hot Water OFF, 3 should be live when Central Heating in ON, 2 & 5 should be Live all of the time. This obviously only checks the NEST is working ok, it doesn't confirm that all is ok from the wiring centre.

 

[CBW]

If you don’t trust either of the 2 engineers, probably best getting someone in familiar with both of those controls.

 

[vikdab]

The link you have included describes my set up 100 %.
I'm not sure as to when exactly the problem started as the mid posn valve and heatlink were replaced at similar time and as luck would have it the engineers blame the other one.

I will check the NEST wiring as suggested above. Thanks for looking in to it

 

[muffking]

It's a decent system, but not as straight forward as others, so it confuses a lot of heating engineers.
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The state of your wiring centre probably isn't helping the engineers diagnosis though. It really needs a proper tidy up and to make sure that the terminal block follows the same order of the diagram, plus I tend to label the cables to make fault finding easier.
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Once you've confirmed that the Nest is working ok then you can test the 3-port valve. Basically turn the HW & CH ON at the Nest and measure the voltages on the cables to the 3-port valve as you turn the room stat & cylinder stat up & down to confirm that they are present at the valve.

 

[vikdab]

Hi

I agree the wiring looks like a complete mess.

I did some tests

On the NEST (on / off refers to the status in app)

Heat Off HW off

1,2,4 and 5 are live

Heating on HW off

2,3,4 and 5 are live

Heating off HW on*

1,2,5,6 are live

Both on

2,3,5,6 are live

Above seems to suggest NEST is connected ok.

Then I checked the status of wires that go to mid position valve (it is a mid position valve not a diverter)

Heating off HW off

only Grey is live

Heating on HW off

Grey/White/Red are live

Heating off HW on

Red live

Both are on

White and Red live


I didn't notice anything wrong with above.

Then I thought maybe it makes a difference on how you get to a position e.g was heating on when you turned hot water on

On the Nest app if I have both heating and hot water on then turn heating off so just hot water is on then
1,2,3,5 and 6 was live. I'm not sure why 3 was live when i was not calling for heat.

My hot water does not work if i turn hot water on like this. I have to turn heating off first.

Then I checked the status of wires to 3 lever valve . Again (as above) if I have both on and turn heating off then only Red wire is live (so white is not live)


Any idea what is wrong here?

 

[muffking]

On the NEST (on / off refers to the status in app)

Heat Off HW off
1,2,4 and 5 are live

Heating on HW off
2,3,4 and 5 are live

Heating off HW on*
1,2,5,6 are live

Both on
2,3,5,6 are live

Based on those readings I can confirm that the NEST is outputting ok in all heating modes.

On the Nest app if I have both heating and hot water on then turn heating off so just hot water is on then
1,2,3,5 and 6 was live. I'm not sure why 3 was live when i was not calling for heat.

I suspect a red herring.
I bet if you were to disconnect the wire at terminal 3 under this condition then the wire would be live, but terminal 3 itself would not be. This is a back feed from the valve. You can't really test this properly though as it would mean disconnecting the cable while it's live.

Then I checked the status of wires that go to mid position valve (it is a mid position valve not a diverter)

Heating off HW off
only Grey is live

Heating on HW off
Grey/White/Red are live

Heating off HW on
Red and White live

Both are on
White and Red live

Yes to the 3-port valve being a mid-position valve.

Heating off HW on
Red and White live = Should be no voltage on white. With your measurements the valve is open for Heating AND Hot Water.

Both are on
White and Red live = This looks to be the same as your Heating off HW on, but ok in this case.

My hot water does not work if i turn hot water on like this. I have to turn heating off first.

Then I checked the status of wires to 3 lever valve . Again (as above) if I have both on and turn heating off then only Red wire is live (so white is not live)

Any idea what is wrong here?

I think your problem is with the wiring to the valve (possibly in the wiring centre) or at the valve connector itself when the new valve was fitted.
Unfortunately it's something that needs to be looked at properly in person as there is no obvious error that stands out.

The only other thing I can suggest is if the 3-port valve is on a connector, can you power down the system and open both ends of the connector up to make sure that all wires are in and maybe make a note of which colours on the valve are connected to what colour on the cable that goes to the wiring centre.

 

[vikdab]

Hi

I checked them again. Heating off HW on only - white did not have any voltage (i have edited my post above) - apologies my mistake

I was wondering why the mid position valve was not behaving as it should.

I replaced the new one with the old one (The old one was replaced because the heating was on switching off at night on very cold days - I think it was because the mid position valve was getting too hot and not operating - the frost thermostat is not connected)

With the old one connected back again HW does come one when heating is on.

This led me to believe that the new one is either faulty or intended to work differently.
My old valve was V4073A1039. New one is 40004805-001 . It says Sundia Y Plan 3 Position Diverter valve

Either the new valve is not a like for like replacement or faulty

Thanks to muffking for the advise

 

[muffking]

No problem.
Does that mean the system is working ok for the moment with the old motorised head back on? If so, then there may well be a difference between the two models. I always try to replace like for like just in case of compatibility issues.

 

[vikdab]

Yes it works now. AT least the hot water bit
However the reason it was replaced was that my heating was not turning off by itself at night (even though heating/hot water off in app)
That problem has resurfaced! Didn't have that with the replacement

I bought it as a like for like replacement on heating control online. It's going back to them

 

[muffking]

It does sound like the motorised head works differently then.
Fingers crossed that the new one fixes it properly when it arrives.

 

[vikdab]

I got new actuator - the right one (40003916-003/U) . The manuals do make a reference to v4073a1039 so I'm sure it is the right one.

However my CH does not turn off when I switch it off in the app (when there is no HW on).
If HW and CH are on and I turn CH off it does work

I think my NEST is connected ok

App status Nest wiring status Mid posn valve wires

Both off 1245 live Grey live

Ch on 2345 White/Grey Live

HW on 1256 Neither live

Both on 2356 White live

I also notice some spurious voltages for eg after I turn Ch off 3 on nest has 55v (white wire as well)
When both CH and HW are on 4 on nest has 72v (also Grey wire)


Any clues on what is going wrong here

 

[muffking]

Both of those Nest and valve head voltages are still ok.

Don't worry about the the low voltage readings as they are reduced voltages where power goes through a diode and a couple of resistors in the valve head so you don't get the full AC waveform or voltage in certain modes. You're only interested in the 240v measurements to test the switching of the valve position.

My gut feeling is that you still have the wrong valve head fitted. You mentioned the part number V4073A1039 as being the valve head that worked ok previously. If that's the case then you need to track down another one, otherwise you have mismatched valve head and valve body. The only other thing to check is that the spindle on the valve body moves freely in both directions by hand with the head removed (just on the off chance that the valve body is sticking and isn't moving under the power of the return spring in the head).

 

[vikdab]

I know the part is correct because it came with Honeywell documentation which says 40003916-003 is correct Replacement head.
Strangely I did not have this problem when I had 40004805-001 . With that one the HW would not come on when CH was on

The spring does bring lever back to mid position when there is no power (I switch mains off)
When I turn power on with CH/HW off on app and force the lever all the way to left (feel spring resistance) the boiler comes on and valve remains open (loose lever)

Will take head out and test spindle the spring does bring lever back to mid position when there is no power

 

[muffking]

I see what you mean with the new 40003916-003 valve head being compatible with the V4073A series body. There's nothing I can think of which stands out. It is strange how 2 slightly different heads are creating 2 different problems though. At this rate I think you're going to need an Electrician who understands Y-Plans to visit and see the fault in the flesh so to speak as it's much easier in person than me trying to figure it out on here.

Just going back to the original valve head V4073A1039 (the one where the heating was switching off at night on very cold days). How did you get on with this when you refitted it?